Children born with certain congenital heart lesions that include obstruction of blood flow to the lungs frequently require that a graft, or "shunt" be placed between the systemic arterial system and the lungs to supply blood to the lungs. The first such operation was performed in 1945 by Alfred Blalock and Helen Taussig using the native left subclavian artery as the graft material. It was a landmark in the treatment of complex congenital heart disease. Since then, prosthetic materials, particularly polytetrafluoroethylene (also known as "PTFE" or Gortex.RTM., a trademark of W.L. Gore & Associates, Inc., Newark, Del., USA, have replaced harvesting of a native artery as the main graft material used for this operation. The operation is performed alone or in combination with other procedures in children from the newborn period to the second decade of life. Such shunts are commonly referred to as "modified Blalock-Taussig shunts."
Despite the current trend toward definitive one stage correction for a variety of congenital cardiac lesions, shunts are still employed in lesions such as tetralogy of Fallot with anomalous coronary anatomy, transposition of the great vessels with ventricular septal defect and left ventricular outflow obstruction, pulmonary atresia with intact ventricular septum, pulmonary atresia with VSD that cannot be closed in one stage, hypoplastic left heart syndrome, complex single ventricle anatomy with pulmonary stenosis, and other lesions in neonates born too small to undergo total one stage correction. Additionally, shunts or "fenestrations" between the systemic venous circuit and the pulmonary venous atrium are now carried out in a substantial fraction of modified Fontanu operations performed worldwide.
Together, these anomalies result in the placement of approximately 3000 shunts annually in the United States, and another 3000 worldwide. To install a typical modified Blalock-Taussig shunt, the surgeon exposes the mediastinum (the contents of the middle of the chest between the two lungs) by dividing the sternum. This is the most common access for all heart operations. The pulmonary artery and a major artery (for example, the subclavian artery) are exposed. The distance between them is measured, then a segment of PTFE graft of appropriate diameter is cut to this measured length. The major artery is clamped, and an opening made in the artery. One end of the graft is then sewn to the artery in fluid communication with the opening. The pulmonary artery is then clamped, and an opening made in it. The other end of the graft is then sewn to the pulmonary artery in fluid communication with the opening. The clamps are then released to establish flow in the graft, going from the major artery to the pulmonary artery (and hence to the lungs).
While performing the operation, the surgeon must make a judgment as to how large the diameter of the graft should be to provide adequate blood flow. Unfortunately, the ideal diameter for the graft generally cannot be predicted with any accuracy. If the graft diameter is too small, the patient will become too cyanotic (blue) since he or she will not have adequate oxygen in the bloodstream. Thus, an undersized graft can result in death in certain cases. If, on the other hand, the graft diameter is too large, the heart will pump too much blood through the lungs (i.e., more than is needed), causing the heart to overwork and fail and death can result. In both situations the patient can become unstable soon after operation, and a number of deaths occur each year as a result. Attempts to control a situation of "undercirculation" or "overcirculation" with drugs achieve only modest success.
Periodically, the surgeon must exchange the graft for one of a different diameter, usually in an emergency situation requiring a sternotomy in the Intensive Care Unit because there is insufficient time to reach an operating room. Every year there are such instances in which the patient does not survive this intervention. Even if the patient survives the early post-operative course, the shunt flow can become inappropriate at a later time, perhaps weeks later, causing heart failure. Additionally, as the patient grows, the shunt flow can become inadequate for the patient's size. On average, a Blalock-Taussig shunt is used for a period of days to weeks, and is typically removed at the child's definitive operation, which usually occurs within the first twelve to eighteen months of the child's life.
In another clinical situation, that of the child with a single ventricle, an operation called a "modified Fontan" is performed, in which a conduit is placed between the inferior vena cava and the pulmonary artery. Because the hemodynamic response to this operation is somewhat unpredictable, the surgeon frequently places a secondary shunt between the conduit and the common atrial chamber. This shunt is often created using polytetrafluoroethylene material, such as Gortex.RTM., and hence resembles the modified Blalock-Taussig graft. It is frequently desirable to regulate the flow through this graft, and eventually totally occlude it.
In accordance with previously known methods, the shunt used in the modified Fontan technique has been partially occluded by snaring the shunt with heavy suture material either brought out through the skin, or buried just beneath the skin. In the latter case, when the surgeon wishes to regulate the flow, he incises the skin in a reoperation in which the snare is exposed, and tightens down on the snare, thus occluding the graft. This procedure requires a reoperation each time the flow needs adjustment and is expensive, risky and labor intensive.
The morbidity and cost of the current imprecision in regulating shunt flow in infants is considerable. In a recent case involving an institution where approximately 580 pediatric cardiac cases are performed per year, 10 procedures in which shunts were used or considered for use were performed during a three month period. In three cases emergency reoperations were required in the intensive care unit while attempting to save the patient by creating or adjusting shunt flow. In three other cases, multi-organ failure and hemodynamic instability were caused by inappropriate shunt flow.
This experience, in which significant morbidity and cost attended 60% of shunt-related procedures, highlights the absence in the field of apparatus and methods for conveniently and accurately adjusting shunt flow. Currently, there are no known "minimally invasive" techniques (i.e., that avoid the necessity of reoperation) for adjusting blood flow through these types of shunts.
Devices are known for regulating the flow of blood within native arteries and fistulas. For example, Edmunds et al. U.S. Pat. No. 3,730,186 describes an implantable pulmonary artery band including an toroidal balloon occluder that is disposed around the native pulmonary artery. The balloon occluder is inflated via a subcutaneously implanted injection button using a conventional hypodermic needle.
A drawback of the Edmunds et al. device is the lack of a mechanism to accurately determine the degree of constriction caused by the balloon occluder. Moreover, the toroidal shape of the balloon occluder is believed to create crimps or infolds in the arterial wall even at low degrees of constriction. Such crimps or infolds, which project into the flow field of the artery, are expected to disrupt laminar flow within the artery and serve as thrombogenic sites.
Lane et al. U.S. Pat. No. 4,828,544, shows a blood control device having a balloon mounted on a strap which is fastened around a fistula. The interior of the balloon is coupled to a pump which is implanted within the patient along with the strap and balloon. Actuating the pump inflates the balloon and restricts the flow of blood through the fistula.
A significant drawback of the device described in the patent to Lane et al. is that it does not permit accurate determination of the degree of constriction introduced in the fistula, nor can it provide real-time measurement of the flow of blood through the fistula.
In view of the foregoing, it would be desirable to provide implantable apparatus for selectively restricting blood flow through a vascular graft, so as to provide precise control over the amount of blood flow through the graft. It further would be desirable to provide methods and apparatus adapted for connection to the implantable apparatus to provide precise real-time external measurement and control over the implantable apparatus. It also would be desirable to provide implantable apparatus that permits constriction of a vascular graft without infolding or crimping of the graft material, thereby reducing the potential for the development of turbulent flow.
Subcutaneous ports are known that provide vascular access for patients needing chronic intravenous drug administration, such as antibiotics, chemotherapy, or blood transfusions. Such ports are made by several companies, for example, the Infuse-A-Port.TM. and DualPort.TM. products made by Infusaid, Inc., Norwood, Mass., USA, and the Hickman ports made by Davol, Inc., Cranston, R.I., USA. The lowest profile port is the CathLink.RTM. 20 by Bard Access Systems, Salt Lake City, Utah, USA.
Access ports permitting electrical connection to an implantable device are also known. For example, Soukup et al. U.S. Pat. No. 5,205,286 and Moden et al. U.S. Pat. No. 4,941,472 show subcutaneous implants having a plurality of electrical access ports. Each access port accepts a single electrical connection. A drawback of the devices described in the foregoing patents is that a separate needle and needle stick is required for each electrical connection, thus increasing patient discomfort and the risk of infection.